In optical communications networks, optical communications modules are used to transmit and/or receive optical signals over optical fibers. Optical receiver modules are optical communication modules that receive optical signals, but do not transmit optical signals. Optical transmitter modules are optical communication modules that transmit optical signals, but do not receive optical signals. Optical transceiver modules are optical communication modules that transmit and receive optical signals.
An optical transmitter or transceiver module has a light source that is driven by a driver circuit to cause the light source to generate amplitude and/or phase and/or polarization modulated optical signals that represent data. The modulated optical signals are optically coupled onto an end of an optical fiber by an optics system of the module. The light source is typically a laser diode or light emitting diode (LED). The optics system typically includes one or more reflective (e.g., mirrors), refractive (e.g., lenses) and/or diffractive (e.g., gratings) elements.
An optical receiver or transceiver module includes a photodetector (e.g., a p-doped-intrinsic-n-doped (PIN) diode) that detects an optical data signal passing out of an end of an optical fiber and converts the optical data signal into an electrical signal, which is then amplified and processed by electrical circuitry of the module to recover the data. An optics system of the module optically couples the optical data signals passing out of the end of the optical fiber onto the photodetector.
As the demand for data throughput continues to increase, the data rate, or bandwidth, of optical links is being pushed ever higher. While various transceiver and optical fiber link designs enable the bandwidth of optical fiber links to be increased, there are limitations on the extent to which currently available technologies can increase the bandwidth of an optical link. One way to increase the bandwidth of an optical link is to shorten the response time of the photodetector. A shortened response time can be achieved by manufacturing the photodetector to have a smaller aperture size, and therefore lower capacitance. However, current manufacturing technologies are limited in their ability to achieve a very small aperture size mainly because of the side effects of using small active areas to collect photons. Even if a very small aperture size is achievable, the optics system of the receiver or transceiver module still needs to be capable of tightly focusing the optical data signal passing out of the end of the optical fiber to form a small beam spot on the aperture of the photodetector with achievable assembly processes and robust performance over the range of operating conditions.
The optics system is usually an imaging system formed by refractive lenses. For a given wavelength and a given light propagation medium, the diameter of the beam spot formed by a lens increases linearly with increased focal length of the lens and decreased incoming beam size. Therefore, for an optical element to achieve a decreased beam spot diameter, either the focal length of the lens must be decreased or its diameter must be increased to accommodate the increased incoming beam size. In parallel optical transceiver modules, the lens diameter is often limited due to other system constraints to a maximum diameter of 250 micrometers (microns). In order to achieve a higher data rate without further increasing the lens diameter, the focal length of the lens must be decreased, which requires either that the lens be made of a material having a higher refractive index or that the lens be made to have a larger sagittal depth (sag) value, which introduces aberrations.
Plastic refractive lenses typically used in the field of optical fiber communications have a higher refractive index than lenses made of glass, but plastic lenses also have a higher coefficient of thermal expansion (CTE) than glass lenses. The higher CTE of plastic lenses can lead to problems at extreme temperatures. Glass lenses made by photolithographic processes are more reliable than plastic lenses at extreme temperatures, but photolithography has limitations with regard to controlling the lens shape. Although molding processes can be used to make glass lenses with larger sag values and better controlled shapes, glass lens molding processes are expensive to perform and difficult to scale up for multiple channels.
In addition to the bandwidth requirement for higher data rate communication, it is also important to control optical back-reflection in order to stabilize the optical output from the transmitter. The stability of the transmitted signal determines the bit-error-rate (BER) of the link, which is a key performance metric. In an imaging system made of refractive lenses, the back reflection from the photodetector directly contributes to destabilization of the source, and therefore needs to be carefully managed. One way to suppress the back reflection is to tighten the anti-reflection coating specification on the surface of photodetector, which inevitably increases the cost of the device. Another way to suppress back reflection is to introduce an oblique incident beam to the photodetector by skewing the optics system. This method, however, can significantly complicate the assembly processes, which also introduces higher costs.
Higher data rate applications often imply higher output power from the transmitter. In a single-mode optical fiber link, where the fiber size is substantially smaller than in multimode optical fiber links, a de-magnified focus spot can sometimes cause an overload condition in the photodetector to occur. However, de-magnification is almost always preferred in order to minimize the impact of fiber misalignment. One solution to this dilemma is to use a non-imaging optics system to manipulate the spot size to maintain a certain minimal level while keeping the overall coupling system de-magnified.
A need exists for an optics system that improves link performance by (1) forming a tight focus spot on the photodetector to enable higher bandwidth, (2) manipulating the focus spot size as needed to avoid an overload condition of the photodetector, and (3) managing optical back-reflecton without increasing overall cost. In addition, a need exists for such an optics system that can be manufactured relatively inexpensively and that is reliable over a wide range of temperatures.